Use the designated PYTHON_EXECUTABLE during build.

We were hardcoding "python" as the command to run the swig wrapper
scripts.  We should be using PYTHON_EXECUTABLE instead.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@213354 91177308-0d34-0410-b5e6-96231b3b80d8